1. Field of the Invention
The present invention relates to an incremental rotary encoder which is suitable for surveying instrument such as total stations, theodlites or the like. The present invention also relates to a surveying instrument which incorporates a magnetic incremental rotary encoder.
2. Description of the Related Art
Some conventional surveying instruments such as total stations, theodlites or the like are provided with an incremental rotary encoder as an angle measuring device. For instance, an optical incremental rotary encoder is used to measure horizontal or vertical angles. FIG. 10 shows a conventional optical incremental rotary encoder. This optical incremental rotary encoder is provided with a main scale 101, a sub-scale 103, an LED (light source) 105, a collimating lens 107 and a photosenor (detector) 109. The main scale 101 and the sub-scale 103 are positioned between the collimating lens 107 and the photosenor 109. In this optical incremental rotary encoder, each of the two scales 101 and 103 is made of glass, which makes the weight of the encoder heavy. Furthermore, in this optical incremental rotary encoder, since the main scale 101 and the sub-scale 103 are arranged separately from each other in the axial direction (the vertical direction as viewed in FIG. 10) while the light source 105 and the collimating lens 107, and the detector 109 need to be positioned on the opposite sides of the main scale and the sub-scale 101 and 103, the space in which the optical incremental rotary encoder is disposed needs to be wide in the axial direction thereof.
Similar to the optical incremental rotary encoder, a magnetic incremental rotary encoder is also known as an angle measuring device. A magnetic incremental rotary encoder is generally provided, on an outer peripheral surface of a magnetic drum (graduator disc) thereof, with a multi-pole magnetized layer having a plurality of magnetized divisions equally divided by the number of divisions N (xe2x80x9cNxe2x80x9d being a positive integer). The magnetic incremental rotary encoder is further provided with a magnetic sensor positioned to face the multi-pole magnetized layer. This magnetic sensor is provided thereon with, e.g., four magnetoresistor elements which are disposed at equally spaced intervals whose pitch is smaller than that of the plurality of magnetized divisions of the multi-pole magnetized layer to detect the variation in the resistance values of the four magnetoresistor elements which vary in accordance with the rotation of the magnetic drum to thereby determine the rotational angle of the magnetic drum with high precision corresponding to the pitch of the plurality of magnetized divisions of the multi-pole magnetized layer. The angle of the pitch is determined according to an interpolative calculation.
The error of the surveying instrument due to eccentricity of the graduator disc (protractor disc) is restricted by the Japanese Industrial Standard. Therefore, in a surveying instrument of a high degree of precision, it is necessary to have two magnetic sensors which are positioned on the opposite sides (offset from each other by 180 degrees) of the graduator disc with respect to the axis thereof so as to compensate for error due to the eccentricity of the graduator disc by taking the average of the detected output voltages of the two magnetic sensors.
Surveying instruments are generally required to have a high degree of precision in their functions. However, in a magnetic incremental rotary encoder, the number of the magnetized divisions (the number of divisions N) of the multi-pole magnetized layer cannot be made as many as that of an optical incremental rotary encoder. Accordingly, influence of harmonic distortion within one pitch of the magnetized divisions is large due to dimensional error and/or deviation of the magnetic resistance curve from the ideal value thereof. Moreover, the magnetic incremental rotary encoder has to be made with extreme precision, so that it is necessary to use a large number of magnetic sensors or magnetoresistor elements to compensate for the harmonic distortion which occurs.
An object of the present invention is to provide an incremental rotary encoder in which error due to eccentricity of the graduator disc thereof and also arbitrary-order harmonic distortion can be simultaneously compensated.
Another object of the present invention is to provide a magnetic incremental rotary encoder which can be incorporated in, and is suitable for, a surveying instrument. Another object of the present invention is to provide a surveying instrument incorporating a magnetic incremental rotary encoder.
Other objects will become apparent to one skilled in the art from a reading of the following disclosure and the appended claims.
To achieve the object mentioned above, according to an aspect of the present invention, an incremental rotary encoder is provided which includes a rotary portion, a first sensor and a second sensor, the first and second sensors being arranged so as to be opposite from each other with respect to the axis of the rotary portion. The second sensor is offset from the first sensor so that the phase of output voltage of the second sensor advances or delays with respect to the phase of output voltage of the first sensor by xcfx80/X, wherein xe2x80x9cXxe2x80x9d represents a real number of one or more than one.
In an embodiment, the second sensor is offset from the first sensor so that the phase of output voltage of the second sensor advances or delays with respect to the phase of output voltage of the first sensor by xcex/2n in the case where an n-order harmonic distortion is compensated for, wherein the pitch of the harmonic distortion is xcex/n.
Preferably, third and fourth sensors are provided, independently of the first and second sensors, which are arranged so as to be opposite from each other with respect to the axis of the rotary portion. In this case, the fourth sensor is offset from the third sensor so that the phase of output voltage of the fourth sensor advances or delays with respect to the phase of output voltage of the third sensor by xcex/2m in the case where a m-order harmonic distortion is compensated for, wherein the pitch of the harmonic distortion is xcex/m.
Preferably, the second sensor is provided in the incremental rotary encoder so that a phase difference of the second sensor with respect to the first sensor is adjustable.
In an embodiment, rotary portion includes a magnetic drum which is rotatably supported by a stationary portion of an optical instrument in which the incremental rotary encoder is incorporated, wherein an outer peripheral surface of the magnetic drum includes a multi-pole magnetized layer having a plurality of magnetized divisions equally divided. Each of the first and second sensors includes a magnetic sensor which is fixed to the stationary portion to face the multi-pole magnetized layer. Each of the first and second sensors includes a plurality of magnetoresistor elements which are located at xcex/4 intervals on the each sensor, xe2x80x9cxcexxe2x80x9d representing the pitch of the plurality of magnetized divisions. Error due to eccentricity of the magnetic drum and the n-order harmonic distortion are compensated for at the same time by taking an average of detected outputs of the first and second magnetic sensors.
In an embodiment, the rotary portion includes a magnetic drum which is rotatably supported by a stationary portion of an optical instrument in which the incremental rotary encoder is incorporated, an outer peripheral surface of the magnetic drum including a multi-pole magnetized layer having a plurality of magnetized divisions equally divided. Each of the first, second, third and fourth sensors includes a magnetic sensor which is fixed to the stationary portion to face the multi-pole magnetized layer, wherein each of the first, second, third and fourth sensors includes a plurality of magnetoresistor elements which are located at xcex/4 or xcex (3/4) intervals on the each sensor, xe2x80x9cxcexxe2x80x9d representing the pitch of the plurality of magnetized divisions. An error due to the eccentricity of the magnetic drum and the n-order harmonic distortion are compensated for at the same time by taking an average of detected outputs of the first and second magnetic sensors. The error due to the eccentricity of the magnetic drum and the m-order harmonic distortion are compensated for at the same time by taking an average of detected outputs of the third and fourth magnetic sensors. The error due to the eccentricity of the magnetic drum, the n-order harmonic distortion and the m-order harmonic distortion are compensated for at the same time by taking an average of detected outputs of the first, second, third and fourth magnetic sensors.
Preferably, the second magnetic sensor is provided in the incremental rotary encoder so that a phase difference of the second magnetic sensor with respect to the first magnetic sensor is adjustable.
Preferably, an adjusting mechanism is further provided which allows the second magnetic sensor to move the second magnetic sensor on a circle about an axis of the magnetic drum along the multi-pole magnetized layer to adjust a phase difference of the second sensor with respect to the first sensor.
Preferably, the phase difference is adjusted to make the sum of a plurality of harmonic distortions minimal in the case of compensating for the plurality of harmonic distortions.
According to another aspect of the present invention, a surveying instrument is provided, which includes a leveling board, a pedestal coupled to the leveling board to be rotatable about a vertical axis relative to the leveling board, a collimating telescope coupled to the pedestal to be rotatable about a horizontal axis relative to the pedestal, a horizontal-angle measuring device for measuring an angle of rotation of the pedestal relative to the leveling board, and a vertical-angle measuring device for measuring an angle of rotation of the collimating telescope relative to the pedestal. At least one of the horizontal-angle measuring device and the vertical-angle measuring device includes an incremental rotary encoder. The incremental rotary encoder includes a rotary portion, a first sensor and a second sensor. The first and second sensors being arranged so as to be opposite from each other with respect to the axis of the rotary portion. The second sensor is offset from the first sensor so that the phase of output voltage of the second sensor advances or delays with respect to the phase of output voltage of the first sensor by xcfx80/X, wherein xe2x80x9cXxe2x80x9d represents a real number of one or more than one.
According to this structure, a conventional surveying instrument can be made light-weight while consuming less space in the surveying instrument, so that the problem of the conventional surveying instrument being heavy to carry and bulky can be improved.
In the case where the magnetic drum is provided on an outer peripheral surface thereof with a multi-pole magnetized layer having a plurality of magnetized divisions equally divided by the number of divisions, it is preferable that the pitch of the plurality of magnetized divisions be represented by the following equation:
60 less than L less than 250(xcexcm)
wherein xe2x80x9cLxe2x80x9d represents the pitch of the plurality of magnetized divisions.
If the pitch L becomes shorter than 60 xcexcm, the positions of the magnetic sensors relative to the magnetic drum have to be minutely adjusted with extreme precision and at the same time the magnetoresistor elements mounted on each magnetic sensor have to be arranged thereon with extreme precision, which makes it difficult to produce the magnetic incremental rotary encoder. On the other hand, if the pitch L becomes greater than 250 xcexcm, in the adjusting operation in which the positions of the magnetic sensors are adjusted while monitoring the Lissajous""s figures (or Bowditch curves) of the output values of the magnetic sensors on the screen of an oscilloscope, the occurrence of a slight variation in the Lissajous""s figures causes the error to vary largely, so that the difference among the Lissajous""s figures cannot be visually determined. This makes it impossible to perform the adjusting operation with high precision. Namely, even if the magnetic incremental rotary encoder is passed by a visual inspection in which the Lissajous""s figures are visually inspected, it is often the case that the error has not been yet sufficiently compensated for.
Preferably, the second sensor is offset from the first sensor so that the phase of output voltage of the second sensor advances or delays with respect to the phase of output voltage of the first sensor by xcex/2n in the case where an n-order harmonic distortion is compensated for, wherein the pitch of the harmonic distortion is xcex/n.
Preferably, third and fourth sensors are further provided, independently of the first and second sensors, which are arranged so as to be opposite from each other with respect to the axis of the rotary portion. The fourth sensor is offset from the third sensor so that the phase of output voltage of the fourth sensor advances or delays with respect to the phase of output voltage of the third sensor by xcex/2m in the case where (m-order harmonic distortion is compensated for, wherein the pitch of the harmonic distortion is xcex/m.
Preferably, the second sensor is provided in the incremental rotary encoder so that a phase difference of the second sensor with respect to the first sensor is adjustable.
Preferably, the rotary portion includes a magnetic drum which is rotatably supported by a stationary portion of an optical instrument in which the incremental rotary encoder is incorporated, wherein an outer peripheral surface of the magnetic drum includes a multi-pole magnetized layer having a plurality of magnetized divisions equally divided. Each of the first and second sensors includes a magnetic sensor which is fixed to the stationary portion to face the multi-pole magnetized layer. Each of the first and second sensors includes a plurality of magnetoresistor elements which are located at xcex/4 intervals on the each sensor, xe2x80x9cxcexxe2x80x9d representing the pitch of the plurality of magnetized divisions. Error due to eccentricity of the magnetic drum and the n-order harmonic distortion are compensated for at the same time by taking an average of detected outputs of the first and second magnetic sensors.
Preferably, the rotary portion includes a magnetic drum which is rotatably supported by a stationary portion of an optical instrument in which the incremental rotary encoder is incorporated, an outer peripheral surface of the magnetic drum including a multi-pole magnetized layer having a plurality of magnetized divisions equally divided. Each of the first, second, third and fourth sensors includes a magnetic sensor which is fixed to the stationary portion to face the multi-pole magnetized layer. Each of the first, second, third and fourth sensors includes a plurality of magnetoresistor elements which are located at xcex/4 or xcex(3/4) intervals on the each sensor, xe2x80x9cxcexxe2x80x9d representing the pitch of the plurality of magnetized divisions. An error due to the eccentricity of the magnetic drum and the n-order harmonic distortion are compensated for at the same time by taking an average of detected outputs of the first and second magnetic sensors. The error due to the eccentricity of the magnetic drum and the m-order harmonic distortion are compensated for at the same time by taking an average of detected outputs of the third and fourth magnetic sensors. Error due to the eccentricity of the magnetic drum, the n-order harmonic distortion and the m-order harmonic distortion are compensated for at the same time by taking an average of detected outputs of the first, second, third and fourth magnetic sensors.
Preferably, the second magnetic sensor is provided in the incremental rotary encoder so that a phase difference of the second magnetic sensor with respect to the first magnetic sensor is adjustable.
Preferably, an adjusting mechanism is provided which allows the second magnetic sensor to move the second magnetic sensor on a circle about an axis of the magnetic drum along the multi-pole magnetized layer to adjust a phase difference of the second sensor with respect to the first sensor.
Preferably, the phase difference is adjusted to make the sum of a plurality of harmonic distortions minimal in the case of compensating for the plurality of harmonic distortions.
Preferably, the pitch of the plurality of magnetized divisions is represented by the following equation:
60 less than L less than 250(xcexcm)
wherein xe2x80x9cLxe2x80x9d represents the pitch of the plurality of magnetized divisions.
Preferably, the magnetic drum is supported to be rotatable about one of the vertical and horizontal axes, each of the magnetic sensors being fixed to one of the leveling board and the pedestal to face the multi-pole magnetized layer.
Preferably, the surveying instrument further includes a vertical shaft fixed to the leveling board, at least one support formed on the pedestal, and a horizontal shaft supported by the at least one support. The pedestal is supported by and rotatable about the vertical shaft. The magnetic drum rotates together with the at least one support. The collimating telescope is supported by the at least one support via the horizontal shaft.
Preferably, the surveying instrument further includes at least one support formed on the pedestal, and a horizontal shaft fixed to the at least one support. The magnetic drum is supported by the horizontal shaft to be rotatable about the horizontal shaft. The collimating telescope is supported by and rotatable about the horizontal shaft wherein the magnetic drum is rotatable together with the collimating telescope. Each of the magnetic sensors is fixed to the at least one support to face the multi-pole magnetized layer.
The present disclosure relates to subject matter contained in Japanese Patent Applications Nos.11-123075 (filed on Apr. 28, 1999) and 11-301115 (filed on Oct. 22, 1999) which are expressly incorporated herein by reference in their entireties.